Graphene nanoribbon precursors and monomers suitable for preparation thereof

ABSTRACT

Provided are graphene nanoribbon precursors comprising repeated units of the general formula (I) in which R 1 , R 2  are each H, halogen, —OH, —NH 2 , —CN, —NO 2  or a hydrocarbyl radical which has 1 to 40 carbon atoms and may be linear or branched, saturated or unsaturated and mono- or poly-substituted by halogen (F, Cl, Br, I), —OH, —NH 2 , —CN, and/or —NO 2 , where one or more CH 2  groups may also be replaced by —O—, —S—, —C(O)O—, —O—C(O)—, —C(O)—, —NH— or —NR—, in which R is an optionally substituted C 1 C 40 -hydrocarbyl radical, or an optionally substituted aryl, alkylaryl or alkoxyaryl radical.

The invention relates to graphene nanoribbon precursors, to graphenenanoribbons obtainable therefrom by oxidative cyclodehydrogenation(intramolecular Scholl reaction), to processes for preparing thegraphene nanoribbon precursors, to monomers suitable for preparation ofthe graphene nanoribbon precursors, and to a process for preparing themonomers.

Graphene nanoribbons (GNRs) are a defined section from the structure ofgraphene. They consist of monolayer ribbons of sp²-hybridized carbonatoms arranged in a honeycomb and have a high side ratio oflength:width, such that they are a quasi-one-dimensional carbonpolymorph. Due to the low width of the ribbons in relation to theirlength, the influence of the edge structure on the electronic propertiesof the graphene cannot be neglected in graphene nanoribbons. Through theedge structure, it is possible to influence the electronic properties ofgraphene nanoribbons in a controlled manner.

Graphene itself has already been used in organic electronics, forexample as a transparent electrode material or as an active material infield-effect transistors. Graphene, however, does not have a naturalband gap, which opposes use as a semiconductor in electronics circuits.However, it has been shown by theoretical models that it is possible ingraphene nanoribbons, by controlling the width and the edge structure,to obtain a synthetic band gap. In order to obtain such semiconductivegraphene nanoribbons, defect-free graphene ribbons of defined structurewith an “armchair” edge structure and a width of <10 nm are needed.These have not been available to date.

It is not possible by “top-down” methods, such as the reduction ofgraphene oxide (S. Stankovich, D. Dikin, R. Piner, K. Kohlhaas, A.Kleinhammes, Y. Jia, Y. Wu, S. Nguyen, R. Ruoff, Carbon 2007, 45, 1558),lithography (M. Han, B. Özyilmaz, Y. Zhang, P. Kim, Phys. Rev. Lett.,2007, 98, 206805, Z. Chen, Y. Lin, M. Rooks, P. Avouris, Physica E,2007, 40, 228), the unzipping of carbon nanotubes (a) L. Jiao, X. Wang,G. Diankov, H. Wang, H. Dai, Nat. Nanotechnol. 2010, 5, 321; b) D.Kosynkin, A. Higginbotham, A. Sinitskii, J. Lomeda, A. Dimiev, B. Price,J. Tour, Nature 2009, 458, 872) or mechanical exfoliation of graphene(X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, Science 2008, 219, 1229), tocontrol the size and edge structure of the graphene nanoribbonsobtained. An organic “bottom-up” synthesis, in contrast, allowsstructural control at the atomic level and is thus suitable forproducing GNRs with exactly defined structure.

X. Yang, X. Dou, A. Rouhanipour, L. Zhi, H. Räder, K. Müllen, J. Am.Chem. Soc. 2008, 130, 4216 disclose the production of a graphenenanoribbon by cyclodehydrogenation of a suitable polymer precursoraccording to Scheme 1 below.

The synthesis is based on the development of a tailored polymerprecursor which is converted to the two-dimensional graphene structurein the last reaction step by oxidative cyclodehydrogenation(intramolecular Scholl reaction). However, full cyclodehydrogenationcould not be achieved, and so a study of the electronic properties wasnot possible due to the presence of defects.

Scheme 2 shows the polymerization to give the polymer precursor.

The maximum length of the polymer at about 10 nm is caused by strongsteric hindrance during the Suzuki polycondensation from the monomers,since the iodine function in the monomer 3 is screened significantly bytwo phenyl radicals in the ortho positions, which makes the couplingreaction more difficult. In addition, thermal scission of thecarbon-iodine bond is easily possible, and causes chain termination. Atthe same time, there is spatial hindrance in the polymer as a result ofoverlapping alkyl radicals, which, in the subsequentcyclodehydrogenation step, hinders formation of aryl-aryl bonds adjacentto these radicals and leads to incomplete cyclodehydrogenation. Anotherdisadvantage is found to be the poly(para-phenylene) structure of thepolymer backbone, which allows only a low level of flexibility along thepolymer chain. This can result in enhanced aggregation and precipitationof the molecules even during the polymerization, before relatively highmolecular weights are attained.

In addition, in the case of polymerization reactions of the A₂+B₂ type,the monomers have to be used in exactly stoichiometric amounts, sinceonly low degrees of polymerization are otherwise achieved.

It is an object of the invention to provide a process for producinggraphene nanoribbons and suitable graphene nanoribbon precursors, whichdo not have the disadvantages of the prior art. It is a particularobject of the invention to provide graphene nanoribbon precursors whichgive defect-free graphene nanoribbons with an “armchair” edge structure.

The object was achieved by graphene nanoribbon precursors comprisingrepeat units of the general formula (I)

in which

R¹, R² are each H, halogen, —OH, —NH₂, —CN, —NO₂, a hydrocarbyl radicalwhich has 1 to 40 carbon atoms and may be linear or branched, saturatedor unsaturated and mono- or polysubstituted by halogen (F, Cl, Br, I),—OH, —NH₂, —CN and/or —NO₂, where one or more CH₂ groups may also bereplaced by —O—, —S—, —C(O)O—, —O—C(O)—, —C(O)—, —NH— or —NR—, in whichR is an optionally substituted C₁-C₄₀-hydrocarbyl radical, or anoptionally substituted aryl, alkylaryl or alkoxyaryl radical,

and the graphene nanoribbons obtainable therefrom by oxidativecyclodehydrogenation.

The object was also achieved by a process for preparing graphenenanoribbon precursors, comprising the Yamamoto coupling reaction ofmonomer units of the general formula (II)

in which

R¹, R² are each H, halogen, —OH, —NH₂, —CN, —NO₂ or a hydrocarbylradical which has 1 to 40 carbon atoms and may be linear or branched,saturated or unsaturated and mono- or polysubstituted by halogen (F, Cl,Br, I), —OH, —NH₂, —CN and/or —NO₂, where one or more CH₂ groups mayalso be replaced by —O—, —S—, —C(O)O—, —O—C(O)—, —C(O)—, —NH— or —NR—,in which R is an optionally substituted C₁-C₄₀-hydrocarbyl radical, oran optionally substituted aryl, alkylaryl or alkoxyaryl radical,

and

X is halogen, trifluoromethylsulfonate or diazonium

and by the monomer units of the general formula (II) themselves.

In general, R¹, R² are each H or a saturated or mono- topentaethylenically and/or -acetylenically unsaturated hydrocarbylradical which may be mono- to pentasubstituted by the substituentsspecified.

Preferably, R¹, R² is H or a linear or branched saturated hydrocarbylradical which may be mono- to pentasubstituted by the substituentsspecified.

Preferably, R¹, R² are each independently hydrogen, C₁-C₃₀-alkyl,C₁-C₃₀-alkoxy, C₁-C₃₀-alkylthio, C₂-C₃₀-alkenyl, C₂-C₃₀-alkynyl,C₁-C₃₀-haloalkyl, C₂-C₃₀-haloalkenyl and haloalkynyl, for exampleC₁-C₃₀-perfluoroalkyl.

C₁-C₃₀-Alkyl may be linear or, if possible, branched.

Examples are methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl,isobutyl, tert-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2,2-dimethylpropyl,1,1,3,3-tetramethylpentyl, n-hexyl, 1-methylhexyl,1,1,3,3,5,5-hexamethylhexyl, n-heptyl, isoheptyl,1,1,3,3-tetramethylbutyl, 1-methylheptyl, 3-methylheptyl, n-octyl,1,1,3,3-tetramethylbutyl and 2-ethylhexyl, n-nonyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl,octadecyl, eicosyl, heneicosyl, docosyl, tetracosyl or pentacosyl.

C₁-C₃₀-Alkoxy groups are straight-chain or branched alkoxy groups, forexample methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, sec-butoxy,tert-butoxy, amyloxy, isoamyloxy or tert-amyloxy, heptyloxy, octyloxy,isooctyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy, tetradecyloxy,pentadecyloxy, hexadecyloxy, heptadecyloxy and octadecyloxy.

The term “alkylthio group” means the same as alkoxy group, except thatthe oxygen atom in the ether bridge has been replaced by a sulfur atom.

C₂-C₃₀-Alkenyl groups are straight-chain or branched, for example vinyl,allyl, methallyl, isopropenyl, 2-butenyl, 3-butenyl, isobutenyl,n-penta-2,4-dienyl, 3-methyl-but-2-enyl, n-oct-2-enyl, n-dodec-2-enyl,isododecenyl, n-dodec-2-enyl or n-octadec-4-enyl.

C₂-C₃₀-Alkynyl is straight-chain or branched, such as ethynyl,1-propyn-3-yl, 1-butyn-4-yl, 1-pentyn-5-yl, 2-methyl-3-butyn-2-yl,1,4-pentadiyn-3-yl, 1,3-pentadiyn-5-yl, 1-hexyn-6-yl,cis-3-methyl-2-penten-4-yn-1-yl, trans-3-methyl-2-penten-4-yn-1-yl,1,3-hexadiyn-5-yl, 1-octyn-8-yl, 1-nonyn-9-yl, 1-decyn-10-yl, or1-tetracosyn-24-yl.

C₁-C₃₀-Perfluoroalkyl is branched or unbranched, such as —CF₃, —CF₂CF₃,—CF₂CF₂CF₃, —CF(CF₃)₂, —(CF₂)₃CF₃ or —C(CF₃)₃.

The terms “haloalkyl, haloalkenyl and haloalkynyl” mean partly or fullyhalogen-substituted alkyl, alkenyl and alkynyl groups.

Aryl is typically C₆-C₃₀-aryl and may optionally be substituted, forexample phenyl, 4-methylphenyl, 4-methoxyphenyl, naphthyl, biphenylyl,terphenylyl, pyrenyl, fluorenyl, phenanthryl, anthryl, tetracyl,pentacyl and hexacyl.

Preferably, X=Cl or Br. More preferably, R¹, R² are each H orC₈-C₃₀-alkyl, especially H or C₁₀-C₂₆-alkyl.

Preferably, R²=H.

Through the Yamamoto coupling reaction proceeding from the inventivemonomer units (II), it is possible to produce graphene nanoribbons withgenerally 3 to 100 and preferably 5 to 50 repeat units (I). The Yamamotopolymerization reaction is additionally not stoichiometry-sensitive likea polymerization reaction of the A₂+B₂ type.

The angled backbone of the graphene nanoribbon precursor moleculereduces steric hindrance during the polymerization step to form thegraphene nanoribbon precursor, and during the subsequentcyclodehydrogenation of the precursor to give the graphene nanoribbon.This allows sterically demanding alkyl radicals to be introduced, whichadditionally induce increased solubility. The relatively high level oftwisting of the angled polymer backbone, which has relatively highflexibility, suppresses the aggregation of the molecules during thepolymerization, as a result of which relatively high molecular weightscan be achieved.

The synthesis scheme for preparation of monomers of the general formula(II) is shown in Scheme 3.

Proceeding from 1,3-di(biphenyl-3-yl)propan-2-one 7, which alreadycomprises the two flexible meta-biphenyl units, Knoevenagel condensationwith 4,4′-dihalobenzil introduces two halogen functions for the laterYamamoto polymerization to obtain the tetraarylcyclopentadienone 8. Thecyclopentadienone 8 is converted by Diels-Alder cycloaddition withoptionally functionalized tolane 10 to give the monomer 6. This reactioncan be performed in a microwave reactor.

The graphene nanoribbon precursors are synthesized from the monomer 6 byYamamoto polymerization in the presence of a nickel catalystcorresponding to Scheme 4. A suitable catalyst system comprisesNi(COD)₂, 1,5-cyclooctadiene and 2,2′-bipyridine in a toluene/DMFmixture as a solvent. The polymers formed can be “end-capped”, i.e. theterminal halogen functions can be exchanged for phenyl, by addition ofchlorobenzene or bromobenzene.

The cyclodehydrogenation of the graphene nanoribbon precursors 11 togive the graphene nanoribbons can be effected by means of intramolecularScholl reaction using, for example, iron(III) chloride as a Lewis acidand oxidizing agent.

In general, the molecular weight of the graphene nanoribbons obtained is2000 to 100 000, preferably 4000 to 50 000, these molecular weightsbeing determinable by means of GPC.

Graphene nanoribbons can also be produced on metal surfaces. This isdone by depositing the monomer on the surface by sublimation. This givesrise to diradicals which are polymerized by a temperature increase togive the graphene nanoribbon precursor. In the last step, furtherthermal treatment of the substrate results in the cyclodehydrogenationto give the finished graphene nanoribbons (see Cai, J.; et al. Nature466, 470-473 (2010)).

The invention is illustrated in detail by the examples below.

EXAMPLES

The figures show:

FIG. 1 the superimposed MALDI-TOF mass spectra of the synthesizedmonomers 6 a-c;

FIG. 2 the relevant aromatic region of the ¹H NMR spectra of thesynthesized monomers 6 a-c;

FIG. 3 MALDI-TOF mass spectra of the polymer precursors 11 a-c;

FIG. 4 the Raman spectrum of the GNR 12 b;

FIG. 5 IR spectra of the GNR 12 a, b.

EXAMPLES 1 TO 3 Monomer Synthesis

The synthesis scheme for preparation of monomers of the general formula(II) 6 a to 6 c is shown in Scheme 5.

Proceeding from 1,3-di(biphenyl-3-yl)propan-2-one 7, which alreadycomprises the two flexible meta-biphenyl units, Knoevenagel condensationwith 4,4′-dibromobenzil 9 a or 4,4′-dichlorobenzil 9 b usingtetrabutylammonium hydroxide as a base introduces two halogen functionsfor the later Yamamoto polymerization. The tetraarylcyclo-pentadienones8 a and 8 b could not be removed from the reactants by columnchromatography, but selected precipitation of the products was possiblefrom DCM in methanol. Thus, 8 a was obtained with a yield of 77%, and 8b with 53%, as violet solids. In the last reaction step, thesolubility-imparting groups were introduced and the cyclopentadienoneswere converted by Diels-Alder cycloaddition with functionalized tolanesto give the target compounds. Due to the high steric demands, thisreaction had to be performed in a microwave reactor at 220° C. at 300watts and over a reaction time of 24 h. After column chromatographypurification with silica gel and repeated reprecipitation, all monomerswere purified by means of recycling GPC. In spite of an associatedreduction in yield, this high purity was necessary for the achievementof high molecular weights in the polymerization. Monomer 6 a withoutalkyl radicals was thus obtained in 40% yield as a colorless solid.Addition of 8b with 4,4′-didodecyltolane 10 b gave monomer 6 b with ayield of 56%, and a reaction with 4,4′-bis(2-decyltetradecyl)tolane 10 cgave monomer 6 c with 41% yield. Both alkylated products were obtainedas colorless oils.

EXAMPLES Example 1a

2,5-Di([1,1′-biphenyl]-3-yl)-3,4-bis(4-bromophenyl)cyclopenta-2,4-dienone(8 a)

To a degassed solution of 2.84 g of 4,4′-dibromobenzil (7.73 mmol) and2.80 g of 1,3-di(biphenyl-3-yl)propan-2-one (7, 7.73 mmol) in 30 ml oftert-butanol was added, at 80° C., a methanolic tetrabutylammoniumhydroxide solution (1 M, 2.84 ml, 2.84 mmol). The reaction solution wasstirred at 80° C. for 20 minutes and then stopped by adding water.Extraction was effected three times with dichloromethane, and thecollected organic phases were washed with saturated sodium chloridesolution and dried over magnesium sulfate before the solvent wasdistilled off under reduced pressure. The crude product was purified bycolumn chromatography (silica gel, eluent: hexane with 20% DCM) and gave2.85 g of the tetraarylcyclopentadienone 8a as a violet wax (53%, 4.10mmol). Elemental analysis measured: C 70.5; H 3.3% (calculated forC₄₁H₂₆Br₂O: C, 70.9; H, 3.8%); ¹H NMR (700 MHz, d₈-THF) δ=7.51−7.49 (m,4H, CH), 7.49−7.46 (m, 4H, CH), 7.41 (dd, J=8.2, 1.1 Hz, 4H, CH), 7.36(t, J=7.8 Hz, 4H, CH), 7.32 (t, J=8.0 Hz, 2H, CH), 7.29−7.25 (m, 2H,CH), 7.25−7.22 (m, 2H, CH), 7.01−6.99 (m, 4H, CH); ¹³C NMR (175 MHz,d₈-THF) δ=154.28, 141.92, 141.83, 133.58, 132.72, 132.34, 132.30,132.12, 129.99, 129.89, 129.71, 129.53, 128.26, 127.80, 127.21, 126.74,123.92; MS (FD, 8 kV): m/z (%): 693.8 (100) [M⁺] (calculated forC₄₁H₂₆Br₂O: 694.0); Rf (hexane with 6% ethyl acetate)=0.47.

Example 1b

1,2-Bis(4-bromophenyl)-3,6-bis(biphenyl-3-yl)-4,5-diphenylbenzene (6 a)

A degassed solution of 300 mg of2,5-di([1,1′-biphenyl]-3-yl)-3,4-bis(4-bromophenyl)cyclopenta-2,4-dienone(8 a, 0.432 mmol) and 77.0 mg of 4,4′-dibromotolane (0.432 mmol) in 3 mlof diphenyl ether was heated to 230° C. in a microwave reactor at power300 watts and a maximum pressure of 7 bar for 3×12 h. After cooling toroom temperature, the reaction solution was diluted with hexane andpurified by column chromatography (silica, hexane with 6% ethylacetate). After purification by means of recycling GPC and drying underhigh vacuum, 145 mg of monomer 6 a were obtained in the form ofcolorless crystals (40%, 0.172 mmol). Elemental analysis measured: C76.7; H 3.1% (calculated for C₅₄H₃₆Br₂: C, 76.8; H, 4.3%); ¹H NMR (700MHz, d₈-THF) δ=7.30−7.25 (m, 4H, CH), 7.23−7.17 (m, 6H, CH), 7.13 (d,J=1.5 Hz, 1H, CH), 7.12−7.11 (m, 1H, CH), 7.11−7.09 (m, 4H, CH),7.09−7.06 (m, 2H, CH), 6.95 (s, 1H, CH), 6.94 (br s, 2H, CH), 6.93 (s,1H, CH), 6.91 (t, J=4.1 Hz, 2H, CH), 6.89 (d, J=8.0 Hz, 2H, CH), 6.85(d, J=7.0 Hz, 4H, CH), 6.83 (d, J=2.0 Hz, 2H, CH), 6.81 (d, J=1.4 Hz,1H, CH), 6.79 (d, J=8.3 Hz, 2H, CH), 6.78 (s, 1H, CH); ¹³C NMR (125 MHz,d₂-TCE) δ=141.05, 140.74, 140.20, 140.00, 139.44, 139.03, 138.52,133.03, 132.92, 131.30, 131.15, 130.26, 129.92, 129.85, 128.42, 127.17,126.87, 126.62, 126.52, 125.20, 124.15, 120.18, 119.51; MS (MALDI-TOF):m/z (%): 845.1 (100) [M⁺] (calculated for C₅₄H₃₆Br₂: 844.1); Rf (hexanewith 6% ethyl acetate)=0.40; m.p. (° C.): decomposition at >400° C.

Example 2a

2,5-Di([1,1′-biphenyl]-3-yl)-3,4-bis(4-chlorophenyl)cyclopenta-2,4-dienone(8 b)

To a degassed solution of 940 mg of 4,4′-dichlorobenzil (3.37 mmol) and1.22 g of 1,3-di(biphenyl-3-yl)propan-2-one (7, 3.37 mmol) in 20 ml oftert-butanol was added, at 80° C., a methanolic tetrabutylammoniumhydroxide solution (1M, 1.7 ml, 1.7 mmol). The reaction solution wasstirred at 80° C. for 20 minutes, and the reaction then stopped byadding water. The mixture was extracted three times with dichloromethaneand the collected organic phases were washed with saturated sodiumchloride solution and dried over magnesium sulfate, before the solventwas distilled off under reduced pressure. The crude product was purifiedby column chromatography (silica gel, eluent: hexane with 20% DCM) andgave 1.56 g of the tetraarylcyclopentadienone 8 b as a pale violet solid(77%, 2.58 mmol). Elemental analysis measured: C 81.3; H 3.3%(calculated for C₄₁H₂₆Cl₂O: C, 81.3; H, 4.3%); ¹H NMR (700 MHz, d₈-THF)δ=7.52 (s, 2H, CH), 7.50 (d, J=7.8 Hz, 2H, CH), 7.42 (d, J=7.2 Hz, 4H,CH), 7.36 (t, J=7.7 Hz, 4H, CH), 7.32 (m, 6H, CH), 7.27 (t, J=7.3 Hz,2H, CH), 7.23 (d, J=7.8 Hz, 2H, CH), 7.06 (d, J=8.5 Hz, 4H, CH); ¹³C NMR(175 MHz, d₈-THF) δ=199.86, 154.28, 141.91, 141.82, 135.61, 133.14,132.14, 132.10, 129.98, 129.90, 129.70, 129.69, 129.52, 128.27, 127.78,127.20, 126.73; MS (MALDI-TOF): m/z (%): 604.6 (100) [M⁺] (calculatedfor C₄₁H₂₆Cl₂O: 604.1); Rf (hexane with 10% ethyl acetate)=0.47.

Example 2b

1,2-Bis(4-chlorophenyl)-3,6-bis(biphenyl-3-yl)-4,5-bis(4-dodecylphenyl)benzene(6 b)

A degassed solution of 1.84 g of2,5-di([1,1′-biphenyl]-3-yl)-3,4-bis(4-chlorophenyl)cyclopenta-2,4-dienone(8b, 3.03 mmol) and 1.72 g of 4,4′-didodecyltolane (3.34 mmol) in 12 mlof diphenyl ether and 5 ml of propylene carbonate was heated to 230° C.in a microwave reactor at power 300 watts and a maximum pressure of 7bar for 2×12 h. After cooling to room temperature, the reaction solutionwas diluted with hexane and purified by column chromatography (silica,hexane with 6% ethyl acetate). After purification by means of recyclingGPC and drying under high vacuum, 1.85 g of monomer 6 b were obtained asa colorless oil (56%, 1.69 mmol). Elemental analysis measured: C 85.6; H7.9% (calculated for C₇₈H₈₄Cl₂: C, 85.8; H, 7.8%); ¹H NMR (700 MHz,d₈-THF) δ=7.27 (dt, J=7.7, 4.0 Hz, 4H, CH), 7.22−7.16 (m, 6H, CH), 7.10(d, J=10.0 Hz, 4H, CH), 6.94 (t, J=7.7 Hz, 4H, CH), 6.91 (s, 2 Hv), 6.88(d, J=8.4 Hz, 2H, CH), 6.87−6.81 (m, 2H, CH), 6.81−6.77 (m, 2H, CH),6.77−6.70 (br m, 6H, CH), 6.66 (d, J=7.5 Hz, 2H, CH), 2.41−2.28 (m, 4H,α-CH₂₁), 1.44−1.34 (m, 4H, β-CH₂), 1.34−1.03 (m, 36H, —CH₂—), 0.89 (t,J=6.9 Hz, 6H, —CH₃); ¹³C NMR (175 MHz, d₈-THF) δ=142.44, 142.42, 142.27,141.88, 141.74, 140.76, 140.74, 140.68, 140.59, 140.56, 140.14, 140.10,139.09, 139.05, 134.18, 134.14, 134.06, 134.01, 132.56, 132.50, 132.47,132.30, 132.26, 131.69, 131.41, 129.37, 128.20, 128.07, 127.90, 127.88,127.84, 127.81, 127.80, 127.74, 125.14, 125.12, 36.36, 36.33, 33.06,32.37, 32.34, 30.85, 30.81, 30.77, 30.75, 30.61, 30.60, 30.51, 30.00,29.98, 25.94, 25.82, 23.74, 14.62; MS (MALDI-TOF): m/z (%): 1091.0 (100)[M⁺] (calculated for C₇₈H₈₄Cl₂: 1090.6); Rf (hexane with 6% ethylacetate)=0.65.

Example 3

1,2-Bis(4-chlorophenyl)-3,6-bis(biphenyl-3-yl)-4,5-bis(4-(2-decyltetradecyl)dodecylphenyl)benzene(6 c)

A degassed solution of 636 mg of2,5-di([1,1′-biphenyl]-3-yl)-3,4-bis(4-chlorophenyl)cyclopenta-2,4-dienone(8 b, 1.05 mmol) and 895 mg of 4,4′-bis(2-decyltetradecyl)tolane (1.05mmol) in 10 ml of diphenyl ether was heated to 230° C. in a microwavereactor at power 300 watts and a maximum pressure of 7 bar for 2×12 h.After cooling to room temperature, the reaction solution was dilutedwith hexane and purified by column chromatography (silica, hexane with6% ethyl acetate). After purification by means of recycling GPC anddrying under high vacuum, 613 mg of monomer 6 c were obtained as acolorless oil (41%, 0.429 mmol). Elemental analysis measured: C 86.0; H9.7% (calculated for C₁₀₂H₁₃₂Cl₂: C, 85.8; H, 9.3%); ¹H NMR (700 MHz,d₈-THF) δ=7.29−7.25 (m, 4H, CH), 7.20 (m, 6H, CH), 7.14 (s, 1H, CH),7.11 (d, J=10.0 Hz, 3H, CH), 6.95−6.88 (m, 9H, CH), 6.85 (d, J=8.4 Hz,1H, CH), 6.81 (d, J=8.0 Hz, 1H, CH), 6.79 (d, J=7.8 Hz, 2H, CH), 6.77(d, J=7.7 Hz, 1H, CH), 6.73 (d, J=7.8 Hz, 2H, CH), 6.69 (t, J=7.6 Hz,2H, CH), 6.64 (d, J=7.9 Hz, 2H, CH), 2.33−2.24 (m, 4H, α—CH₂), 1.44−1.37(m, 2H, β—CH₂), 1.35−0.95 (br m, 80H, —CH₂—), 0.89 (m, 12H, —CH₃); ¹³CNMR (175 MHz, d₈-THF) δ=142.43, 142.41, 142.20, 142.18, 141.94, 141.77,140.69, 140.65, 140.55, 140.52, 140.10, 140.07, 139.61, 139.08, 134.13,134.04, 133.98, 132.48, 132.31, 132.13, 131.70, 131.37, 129.41, 128.67,128.53, 128.21, 128.07, 127.87, 127.77, 125.08, 40.92, 40.61, 33.94,33.86, 33.81, 33.08, 33.06, 31.20, 31.12, 31.10, 30.89, 30.86, 30.82,30.54, 30.52, 27.49, 27.45, 25.93, 25.82, 23.76, 23.75, 14.63; MS(MALDI-TOF): m/z (%): 1427.8 (100) [M⁺] (calculated for C₁₀₂H₁₃₂Cl₂:1428.0); Rf (hexane with 6% ethyl acetate)=0.76.

FIG. 1 shows the superimposed MALDI-TOF mass spectra of the synthesizedmonomers 6 a-c. It was possible in all three cases to obtain theproducts in pure form and to ensure that no by-products which could havecaused termination of chain growth during the later polymerization werepresent any longer. The exact structure of the monomers was confirmed by¹H NMR spectroscopy.

FIG. 2 shows the relevant aromatic region of the ¹H NMR spectra ofmonomers 6 a-c, recorded in d₈-THF (700 MHz, RT). The ¹H NMR spectra ofmonomers 6a-c can be resolved only with difficulty since all 34 to 36aromatic protons exhibited a very similar chemical shift. The signalsare within a narrow range from 6.6 to 7.3 ppm, and some aresuperimposed. By means of DOSY (diffusion-ordered spectroscopy), thediffusion properties of the molecules in the sample can be determined,and COSY experiments can determine couplings between the signals ofconjugated protons in the NMR.

Examples 4 to 6 Polymer Synthesis

Once the structure and purity of monomers 6a-c had been confirmed, thecorresponding polymers were synthesized by Yamamoto polymerizationaccording to Scheme 6.

Scheme 6 shows the synthesis of the graphene nanoribbon precursors 11a-cby Yamamoto polymerization of the dihalogenated monomers 6 a-c withcatalysis by Ni(COD)2, 1,5-cyclooctadiene and 2,2′-bipyridine intoluene/DMF. The yields were (i) 84%, (ii) 86%, (iii) 67%. Since thecatalytically active nickel(0) reagent in the Yamamoto polycondensationis very sensitive to water and oxygen, all monomer units 6 were driedunder high vacuum before they were used for the polymerization. Thecatalyst mixture composed of 59.4 mg of Ni(COD)₂, 23.4 mg of1,5-cyclooctadiene and 33.7 mg of 2,2′-bipyridine (0.216 mmol of each)was weighed out in a glovebox under an argon atmosphere and preparedtogether with the solvents in a microwave-compatible glass reactionvessel, sealed with a gas-tight aluminum lid with septum and protectedfrom any incident light. The use of a microwave reactor to conduct thereaction gives the advantage of a distinctly increased reaction rate anda reaction temperature above the boiling point of the solvents. Afterthe thermal activation of the catalyst at 60° C. for 20 minutes, adegassed solution of 0.09 mmol of the monomer in 1 ml of anhydroustoluene was introduced into the reaction vessel through the septum, andthe polymerization was performed at microwave power 300 watts and 80° C.over a period of 10 hours. A monomer concentration of about 50 mg/ml wasused to promote the attainment of high molecular weights. To end-cap thepolymers, a degassed solution of bromo-/chlorobenzene in toluene (0.5ml, 0.01 molar) was finally added, and the mixture was heated again to80° C. for 20 minutes. To purify the products and remove catalystresidues, the reaction solution was added dropwise to an HCl/methanolmixture and stirred overnight. The resulting precipitate was removed ina centrifuge and repeatedly reprecipitated with THF in methanol, beforeit was filtered off and dried under reduced pressure. The polymers 11 awithout alkyl radicals and 11 b with dodecyl chains were obtained inyields of 84 to 86% as colorless solids. Only by the introduction ofbranched 2-decyltetradecyl radicals was the melting point lowered tosuch an extent that the polymer 11 c was present as a colorless oil atroom temperature. Before the subsequent cyclodehydrogenation to give thecorresponding GNRs, low molecular weight oligomers were removed by amanual preparative GPC fractionation. This was possible since all threepolymers were fully soluble in common organic solvents such as THF, DCMor toluene.

Example 4

The monomer used was 76.0 mg of1,2-bis(4-bromophenyl)-3,6-bis(biphenyl-3-yl)-4,5-diphenylbenzene (6 a,0.09 mmol). After conclusion of the reaction and cooling to roomtemperature, a colorless precipitate had already formed. Afterpurification of the crude product, 43.7 mg of the polymer 11 a wereobtained as a colorless solid (84%). GPC analysis: Mn=0.11×10⁴ g/mol,M_(w)=0.15×10⁴ g/mol, polydispersity D=1.35 (UV detector, PS standard),DSC (° C.): no transitions.

Example 5

The monomer used was 98.3 mg of1,2-bis(4-bromophenyl)-3,6-bis(biphenyl-3-yl)-4,5-bis(4-dodecylphenyl)benzene(6 b, 0.09 mmol). After conclusion of the reaction and cooling to roomtemperature, the reaction solution had turned dark brown, and there wasa black precipitate on the flask wall. After purification of the crudeproduct, 79.0 mg of polymer 11 b were obtained as a colorless solid(86%). GPC analysis: M_(n)=0.93×10⁴ g/mol, M_(w)=1.25×10⁴ g/mol,polydispersity D=1.34 (UV detector, PS standard), DSC (° C.): notransitions.

Example 6

The monomer used was 128.6 mg of1,2-bis(4-bromophenyl)-3,6-bis(biphenyl-3-yl)-4,5-bis(4-(2-decyltetradecyl)dodecylphenyl)benzene(6 c, 0.09 mmol). After conclusion of the reaction and cooling to roomtemperature, the reaction solution had turned dark brown, and there wasa black precipitate on the flask wall. After purification of the crudeproduct, 81.9 mg of polymer 11 c were obtained as a colorless oil (67%).GPC analysis: M_(n)=0.35×10⁴ g/mol, M_(w)=0.48×10⁴ g/mol, polydispersityD=1.37 (UV detector, PS standard), DSC (° C.): no transitions.

The molecular weights attained in polymers 11 a-c were determined byMALDI-TOF MS and GPC analysis. Since no suitable standard was availablefor a GPC analysis, a polystyrene standard was used due to the angledbackbone of the polymers. MALDI-TOF MS is subject to the limitation thatdetection of high molecular weight species was impossible due to thepolydispersity of the samples. The data obtained here therefore permitonly conclusions about the minimum molecular weights in the sample. TheMALDI-TOF mass spectra recorded for polymer precursors 11 a-c arereproduced in FIG. 3.

The analysis of polymers 11 a-c by means of MALDI-TOF MS showed thatvery regular signal patterns were observed in all cases, for which therewas a high level of correspondence between the spacings of the signalsand the calculated molecular weights of the respective repeat units. Inthe case of 11 a, the intense signals were assigned to the fullydebrominated product. The weak signals arose through adsorption ofsilver ions during the ionization, and were not observed in reflectormode. It was possible to detect molecular weights up to 5000 g/mol,which corresponded to a maximum of seven repeat units. In the case ofpolymer 11 b with dodecyl chains, molecular weights of up to 20 000g/mol (19 repeat units) were detected. In the case of polymer 11 c,seven repeat units with a molar mass up to 10 000 g/mol were detected.

Since all three polymers 11 a-c (apart from the alkyl radicals) had thesame repeat unit, it was easily possible to convert the molecularweights to the chain length. This length corresponded to the laterlongitudinal dimension of the GNRs after the cyclodehydrogenation. Forthe graphene nanoribbon precursor 11 b with dodecyl chains, the molarmass of 20 000 g/mol corresponded to a later graphene ribbon with awidth of 2.1 nm and a length of about 12 nm (˜1.2 nm/repeat unit).

Examples 7 to 9 Cyclodehydrogenation

The cyclodehydrogenation of the polymer precursors 11 a-c to give thecorresponding GNRs 12 a-c according to Scheme 7 was performed by meansof intramolecular Scholl reaction using iron(III) chloride as the Lewisacid and oxidizing agent.

Typically, the reaction was performed with a very low polymerconcentration of 1 mg/ml in unstabilized dichloromethane in order toprevent the occurrence of intermolecular aryl-aryl couplings. Thereaction solutions were degassed with an argon stream over the entirereaction time in order to drive out oxygen and the HCl which formed. Atthe start of the reaction, six equivalents of iron(III) chloride perbond to be formed (90 equivalents per repeat unit) were added rapidly asa concentrated solution in nitromethane, and the mixture was stirred atroom temperature for three days. After the cyclodehydrogenation hadconcluded, the GNRs were precipitated with methanol and purifiedfurther.

GNR 12 a without alkyl radicals and GNR 12 b with dodecyl radicals wereobtained with a yield of 64 and 98% as black solids, which wereinsoluble in standard organic solvents such as toluene, THF,tetrachloroethane or chloroform. With a width of the graphene ribbon of2.1 nm, there were such strong p-p interactions that two dodecylradicals per repeat unit in the case of 12 b were insufficient toprevent aggregation. For workup, the crude products were therefore freedof all soluble impurities by Soxhlet extraction with THF and methanol,and finally dried under high vacuum. GNR 12 c with 2-decyltetradecylchains, in contrast, was obtained in a yield of 81% as a black solid,which was soluble in standard organic solvents such as THF or toluene.The purification was therefore effected by repeated reprecipitation fromTHF in methanol and subsequent Soxhlet extraction with acetone, in orderto remove impurities, by-products products and inorganic residues.

Example 7

50 mg of polymer precursor 11 a were reacted with 1.12 g of FeCl₃ (6.87mmol, dissolved in 4 ml of nitromethane). After a reaction time of threedays, a black precipitate had already formed, which was removed in acentrifuge. For purification, the crude product was in each casesubjected to a two-day Soxhlet extraction with THF and methanol, andfinally dried under high vacuum. Thus, 32.0 mg of the GNR 12 a wereobtained as a black insoluble solid (64%). DSC (° C.): no transitions.

Example 8

76.6 mg of polymer precursor 11 b were reacted with 1.10 g of FeCl₃(6.76 mmol, dissolved in 3.5 ml of nitromethane). After a reaction timeof three days, a black precipitate had already formed, which was removedin a centrifuge. For purification, the crude product was in each casesubjected to a two-day Soxhlet extraction with THF and methanol, andfinally dried under high vacuum. Thus, 72.9 mg of the GNR 12 b wereobtained as a black insoluble solid (98%). DSC (° C.): no transitions.

Example 9

38.11 mg of polymer precursor 11 c were reacted with 410 mg of FeCl₃(2.53 mmol, dissolved in 1.3 ml of nitromethane). After addition of themethanol, a black precipitate formed, which was removed in a centrifugeand freed of all impurities, by-products and inorganic residues byreprecipitation from THF in methanol and subsequent two-day Soxhletextraction with acetone. After drying under high vacuum, 30.2 mg of theGNR 12 c were obtained as a black solid (81%). DSC (° C.): notransitions.

The complete cyclodehydrogenation and the defect-free structure of theGNRs 12 a-c were demonstrated by means of Raman and IR spectroscopy.FIG. 4 shows the Raman spectrum of the GNR 12 b, recorded in a thinpowder film with laser excitation at λ=488 nm. Raman spectroscopyallowed relevant information about the extent of the π-system within theGNRs to be obtained, and thus the conjugation length to be calculated.By IR absorption measurements, it was possible to examine the presenceof a band at 4050 cm⁻¹ for all samples, which was characteristic of thefree rotation of phenyl rings and, in the case of fullcyclodehydrogenation, was no longer detectable. GNR 12 b was the onlysample which showed no fluorescence in the solid state, and thus allowedthe recording of Raman spectra on thin powder films with a laserexcitation wavelength of 488 nm. The spectrum obtained is shown in FIG.4. Good resolution was obtained both for the characteristic D band at1331 cm⁻¹ and the sharp G band at 1579 cm⁻¹. The position of these bandscorresponded to a high degree with values known from literature forgraphene ribbons, which confirmed the graphene character of the sample.At multiples of these wavenumbers, it was also possible to find thesecond- and third-order signals. For a calculation of the dimensionsL_(a) of the GNR 12 b, the ratio of the integrals (I) of first-order Dand G bands was converted by the formula I(D)/I(G)=C(λ)/L_(a). C(λ) wasa wavelength-dependent factor, which assumed the value C(λ)=4.4 nm forλ=488 nm. Thus, a dimension of 4.6 to 4.7 nm was calculated, whichcorresponded to a graphene ribbon with about eight repeat units and amolecular weight of about 8000 g/mol.

The completeness of the cyclodehydrogenation of the GNRs 12 a and 12 bwas additionally confirmed by IR spectroscopy. FIG. 5 shows the IRspectra of the GNRs 12 a and 12 b. The band at 4050 cm⁻¹ wascharacteristic in each case of the free rotation of phenyl rings, and itwas observed clearly in the spectrum of the polymer precursors 11 a and11 b (upper lines). After conclusion of the cyclodehydrogenation,complete absence of this band (lower lines) ruled out the presence ofuncondensed phenyl rings in the molecules, and hence proved the completecyclodehydrogenation.

1. A graphene nanoribbon obtained by cyclodehydrogenation, in solutionor on a metal surfgace, of a precursor comprising repeat units of thegeneral formula (I)

in which R¹, R² are each H, halogen, —OH, —NH₂, —CN, —NO₂ or ahydrocarbyl radical which has 1 to 40 carbon atoms and may be linear orbranched, saturated or unsaturated and mono- or poly-substituted byhalogen (F, Cl, Br, I), —OH, —NH₂, —CN, and/or —NO₂, where one or moreCH₂ groups may also be replaced by —O—, —S—, —C(O)O—, —O—C(O)—, —C(O)—,—NH— or —NR—, in which R is an optionally substituted C₁C₄₀-hydrocarbylradical, or an optionally substituted aryl, alkylaryl or alkoxyarylradical.